1. Field of the Invention
The present invention relates generally to radiographic imaging, of the sort used in nuclear medicine, and more particularly to an improved method and apparatus for maximizing count rate in imaging by Positron Emission Tomography (PET) or Hybrid PET.
2. Description of the Background Art
Radiographic imaging involves the detection of radiation from a distributed radiation field, to form an image. The detection of radiation may lead to information on the structure or on a process of the test subject targeted.
Gamma rays are a form of radiation typically used in radiographic imaging because they have the ability to pass through soft tissues and bones. For these reasons gamma rays are useful in medical imaging, particularly in creating images of a particular organ or an area of interest in the body. This technology provides physicians a non-invasive diagnosis technique to evaluate the performance and function of a targeted area.
To generate these gamma rays a radioactive marker/tracer (radiopharmaceutical) is injected into a test subject. The radioactive marker then travels to the target area, where it is absorbed or retained. As the radioactive marker decays it emits a positron, which soon thereafter collides with an electron. On impact, the positron and electron annihilate each other, generating gamma rays in opposite directions. These gamma rays are predictable and once emitted they can further be detected and used in creating an image.
The radiographic imaging device used to detect the gamma rays are known as gamma ray cameras. Typically gamma cameras utilize a scintillation crystal (usually made of sodium iodide) which absorbs the gamma photon emissions and emits light photons (or light events) in response to the gamma absorption. The amount of, or intensity of the gamma rays detected, is reflected in the emission of the gamma photons. This is known as the scintillation process.
These photons which are more easily detected than radiation, are then detected by a photon detection sensor. Typically the sensors are photomultiplier tubes, which convert the detected photons into a representative electrical signal. These electrical signals are then processed to form an image.
However, the image may not be completely representative of the targeted organ or area. Often, single photon activity from outside the camera field of view (FOV) affects the image. The effect of this activity includes but is not limited to: dead time effects, random coincidence counts and scatter coincidence counts (where coincidence counts are used to measure when two events have occurred within a particular time frame).
In coincidence imaging, septa are aligned within the transaxial planes, normal to the center axis of the cylindrical FOV, to limit the effects of activity outside the camera, such as dead time effects and random coincidence counts. Further, but to a lesser extent, septa may also limit scatter coincidence counts. However, the septa may also reduce the sensitivity to true coincidence counts within the detector. Therefore, there is a tradeoff in septa design, which requires a balance between the reduction of outside activity and the reduction in true coincidence counts, in order to achieve the optimum count rate performance.
The activity concentration (for example patient average) that yields the optimum count rate performance depends on the septa design, and in particular on the septa pitch (septa-to-septa spacing). The Noise Equivalent Count(NEC) rate is one measure of the rate at which the PET camera accumulates good data as a function of the amount of tracer. The peak NEC rate for 3D imaging (no septa), for example, can occur at an activity concentration that is a factor of 3 smaller than for 2D imaging (approximately 1 cm pitch).
In previous implementations, a PET or Hybrid PET camera had the choice of either using one realization or septa spacing, or not using septa at all in coincidence imaging. The point of activity at which the peak NEC rate is achieved, and not the peak NEC rate itself, can substantially be changed due to the interplay of single photon sensitivity, coincidence sensitivity, dead time and scatter fraction scenarios. Further, the patient activity concentration may also vary over a large range, for reasons such as the patient weight, and dose and post-injection imaging time. Therefore, the limited choice of 2D (one septa realization) or 3D (no septa) may not optimize the count rate performance of the camera.
The present invention provides an apparatus and method for optimizing the imaging count rate for either the PET or Hybrid PET camera, using dynamically optimized septa. An apparatus is provided according to the first aspect of the invention, which can employ either inter-plane septa or surface mounted septa. Further, the septa are dynamically adjustable, to achieve optimum coincidence count rate performance. The septa are adjusted based on the findings of detector performance indicators, such as single photon count rate, dead time, or busy time.
According to the second aspect of the invention, a process is described in which detector performance indicators are used to alter the spacing of the septa. The process can be implemented in two forms, either by an automated system or a manual system. In an automated system, the dynamically optimized septa uses the detector indicator performance indicator to automatically adjust the septa spacing, to achieve optimum count rate performance. In an manually adjusted system, the detector performance indicators are used to indicate to the user what the septa spacing is to obtain optimum count rate performance. The user would then manually adjust the septa spacing, as recommended.
The dynamically optimized septa adjusts the septa spacing to optimize the PET or Hybrid camera imaging count rate. The dynamically optimized septa may be implemented in at least three ways to determine the point of peak performance, which include: single photon count rate, detector dead time or detector busy time. As an example, the invention is described in terms of imaging the thorax using single photon count rate as a detector performance indicator.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.